In prior generations of complementary metallic oxide semiconductor (CMOS) technology, shipped product quality level (SPQL) exposure was generally controlled using statistical analyses of chip-to-chip, wafer-to-wafer and lot-to-lot variations resulting from variations in the steps of the process used to fabricate the integrated circuits (ICs). However, as feature size continues to shrink to 90 nm and below with advancing CMOS technologies, across-chip variations in features such as transistor channel length are increasingly affecting SPQL exposure.
One variation of interest in CMOS technology is the performance offset variation between p-type and n-type field effect transistors (PFETs and NFETs, respectively). A parameter often used in conventional PFET-to-NFET device performance analysis is the saturation current of the PFETs and NFETs. During Monte Carlo simulation of the saturation currents (Ion) of a PFET and a corresponding NFET, a scattergram (such as scattergram 100 of FIG. 1) is typically obtained, as are statistics, such as the normalized distribution 200 of FIG. 2. Designers can then use this and other statistical information in performing their circuit analyses.
For example and referring again to FIG. 1, statistics calculated for the PFET and NFET saturation currents can be used to define a set of test screening boundaries 104, here plus and minus three standard deviations (±3σ), that define an acceptable performance envelope 108 that sets the upper and lower bounds on acceptable PFET-to-NFET Ion performance offset. Designers would then use test screening limit boundaries 104 (i.e., the ±3σ values) during testing of as-manufactured ICs to screen-out ICs having PFET-to-NFET device performance mismatch that falls outside acceptable performance envelope 108. Fabricated ICs having actual PFET-to-NFET Ion performance at, for example, points 112 outside acceptable performance envelope 108 would not be shipped, whereas chips having actual PFET-to-NFET Ion performance at points such as points 116 within the acceptable performance envelope would be shipped (assuming other screening measures are satisfied).